1. Field of the Invention
The present invention relates to a pulse radar with a scanning antenna to acquire targets; and, more particularly, to automatic acquisition of targets in an air combat situation prior to tracking the target or lock-on.
2. Description of the Prior Art
Automatic acquisition modes in non-coherent pulse-mode radars are plagued with interference from clutter returns arising from backscatter where the beam impinges upon the ground or from side lobes of the beam impinging the ground approximately below the antenna. Side lobe reflections from the ground below the antenna are especially troublesome because they are present for all pointing directions of the radar antenna in that some side lobe radiation is always reflected by the ground to the antenna. These radar return signals have been called "altitude lines" by radar operators because they cause a line to appear on a B-scope display at a range equal to the actual altitude during azimuth scans of the antenna. As the pilot tries to point his aircraft such that the target is within the antenna search scan pattern, he can get an unwanted radar tracking lock on the altitude line due to its presence before the target is illuminated by the antenna. If the altitude is less than the range or distance to the desired target, the altitude line will be acquired by the radar even if the antenna is illuminating the desired target. This is because the altitude line is interpreted to be greater threat by the automatic acquisition circuitry since it is nearer in range.
Previous attempts at solving the problem of an unwanted radar tracking lock on the altitude line included pulse width discrimination, sensitivity time control and gain tuning, and altitude line tracking and blanking. Of these approaches, only altitude line tracking has been successful. This has involved a special altitude line detection antenna scan and special tracking hardware. This approach adds significant time delays in the target acquisition process and adds to the radar hardware costs and complexity.
It is therefore desirable to provide a means and method for acquiring desired targets while at the same time rejecting the altitude line since the pilot must be confident that his radar has locked on the desired target for the automatic acquisition mode of the radar to be useful in combat.
It is further desirable that all returns from the side lobe, as well as the main beam, are processed by the same circuitry. By using the same circuitry, identical detection sensitivities and range measurement accuracies are maintained for both desired and undesired returns. This overcomes the problems encountered in those methods where separate circuitry is used to detect and track the undesired returns.
It is further desirable to reject target reports that extend over an antenna scan angle that is too large to be a valid target.
It is further desirable to select target reports which extend over a limited scan angle by the antenna indicative of a target having a valid target size.
It is further desirable to use target reports rejected during a prior scan as a basis for rejecting target reports of a current scan.